Testing also established that the model can be flown with a payload of about 40g for about 15 minutes; we deem this sufficient to achieve the project’s aims.

Fig. 1. Helicopter retrofitted with the new electronics, the Gumstix computer and the IMU.

3.1 Electronics and sensors

Since the helicopter is sold as a remotely controlled toy, the first step towards autonomy involved the complete redesign and replacement of the helicopter’s electronics.

The bulk of the work on the platform involved the design and manufacture of an electronic board that interfaces a Gumstix SBC (single board computer) to the two main electric motors powering the rotors, and to the servos controlling the swashplate. The board was manufactured in surface mount technology, and is based around a low power 40 MHz ARM7 microcontroller. The microcontroller offers two serial ports for reflashing and for communication with the Gumstix, two i2c ports to interface to the ultrasonic sensors and the IMU (inertial mea- surement unit), and four PWM outputs to drive motors and servos. External interrupt inputs are also available to interface to two rotor speed encoders; an additional analogue input permits us to monitor the battery voltage. A second companion electronic board accommodates the power stabilisation circuitry and the highly efficient MOSFET motor drivers. The two electronic boards were specifically designed to fit within the original central housing of the helicopter in order to maintain all the moments of inertia as close as possible to those of the original helicopter.

The choice of having an additional low level microcontroller that directly interacts with the hardware was made to guarantee a high degree of reliability for the control system. The absence of an operating system allows a tighter cou- pling with the hardware, offering real-time execution of the critical code needed for helicopter stabilisation. The processing power of the Gumstix SBC will be entirely dedicated to the high level software (e.g. guidance and communication), where the use of the Linux operating system will allow for easy and fast develop- ment. Along with the electronics, a set of low level routines has been developed

to allow the microcontroller to interact with the hardware and enable communi- cation with the Gumstix SBC. Thanks to the Bluetooth wireless communication present on the Gumstix we are now at a stage in which it is possible to com- mand all the helicopter flight controls from a remote computer. A simple yaw stabilization based on the IMU gyros has been implemented to aid during man- ual flight; being able to fly the helicopter manually by exploiting the Bluetooth connection is obviously important to test the helicopter hardware, but will be crucial during the process of data collection for modelling purposes. Previous work of the authors [15] with a similar helicopter and electronics showed how the delay introduced by the Bluetooth connection still allows for the full control of the flight machine. In the final system the communication delay will not be an issue since the control algorithm will run on board.

Ultrasonic sensors and an IMU are the crucial sensors for the control and stabilisation of the vehicle; however, for flocking, each helicopter also needs to determine the range and bearing of its nearby flockmates. Fortunately our in- door arena will shortly be provided with a state of the art 3D tracking system based on infrared markers that will be able to determine with great accuracy (i.e. to within a few millimetres) the position of each helicopter. The relative positions computed by a stationary computer can then be fed back to each of the helicopters through the wireless data link. This will enable us to test the basic flocking algorithm. Further research will investigate the direct sensing of relative position using RSSI (received-signal-strength-indication) from the com- munication channels, the use of onboard radio beacons, and also onboard vision.